This invention relates in general to a transfer apparatus for use for example in an electrographic copier, and more specifically to a roller transfer apparatus of particular construction and the mechanism for applying an electrical bias thereto.
In typical electrographic reproduction apparatus (copiers or copier/duplicators), marking particles are attracted to a latent image charge pattern formed on a dielectric support to develop such image on the support. The dielectric support is then brought into contact with a receiver member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support and the image is fixed to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle transfer is generally accomplished by ion emission from a corona charger onto the receiver member, or by supporting the receiver member on an electrically biased roller holding the receiver member against the dielectric support. While roller transfer apparatus are inherently more complex than corona transfer apparatus, roller transfer apparatus offer certain advantages. For example, roller transfer apparatus typically require a lower energy budget, and also maintain a more positive (physical) control over the receiver member. This positive control is particularly desirable when a receiver member must be recirculated to have multiple images transferred thereto, such as in making multi-color reproductions.
Positive control over the receiver member on the transfer roller has heretofore been provided by mechanical grippers or vacuum mechanisms. Mechanical grippers, such as shown in U.S. Pat. No. 3,612,667 (issued Oct. 12, 1971, in the name of Langdon et al) are of complex construction. For example, the grippers must be recessible within the periphery of the transfer roller to prevent their contacting the dielectric member and causing damage thereto. Vacuum tacking mechanisms are, on the other hand of a much more simple construction (see for example, U.S. Pat. No. 3,633,543, issued Jan. 11, 1972, in the name of Pitasi et al). However, in vacuum tacking of a receiver member to the transfer roller, control over the location of the lead edge of the receiver member for accurate registration of the marking particle images on the dielectric member to the receiver member is not as precisely effectable as with mechanical grippers. Any misregistration may result in the reproduction on such receiver member being of unacceptable quality. Moreover, electrostatic forces of the transfer field generated by the electrical bias applied to the transfer roller tend to attract the receiver member to the dielectric member (that is, the electrostatic forces tend to act in opposition to the vacuum tacking forces. These opposing forces create the potential for preventing accurate tacking of the receiver member to the transfer roller from taking place resulting in undesirable copier stoppages.